Amadori compounds are formed when a reactive substance having an amino group(s) such as protein, peptide or amino acid co-exists with a reducing sugar such as glucose in blood and food product. Thus, they combine together non-enzymatically and irreversibly through the amino group and aldehyde group, which is followed by amadori rearrangement to form an amadori compound. Since the production rate of an amadori compound is a function of concentration of reactive substances, contacting period, temperature, and the like, various information about a sample containing such reactive substances can be obtained from the amount of amadori compound. Therefore, analysis of amadori compounds is useful in the fields related to medicine, food, and the like. In the medical field, attention is particularly focused on the glycated protein as an index for diagnosis and control of conditions of diabetes. Diabetes causes various systemic symptoms (complications) such as diabetic retinopathy, diabetic nephropathy, diabetic neuropathy, and the like, and is the leading cause of blindness and introduction of dialysis. These complications are linked not only to the restriction of daily life and social activity of patients but also to the swelling medical expenses and raise a serious social problem. The importance of early detection and the following adequate control of blood glucose level has been indicated. As an index for controlling blood glucose in diabetes, glycohemoglobin reflecting the mean glucose level for the past about 1 to 2 months, glycoalbumin reflecting the mean glucose level for the past about 2 weeks, or fructosamine corresponding to glycated protein having reducing ability in serum is measured. Glycohemoglobin (HbA1c) is a glycated hemoglobin wherein α-amino group of valine at N-terminus of hemoglobin β chain is glycated. The measurement of HbA1c plays an important role in control of blood glucose level of diabetic patients.
The determination of amadori compound in enzymatic assay is carried out by contacting an amadori compound with an oxidoreductase, and measuring the amount of hydrogen peroxide produced or that of oxygen consumed. Fructosylamino acid oxidase, one of oxidoreductases, has generally been purified from microorganisms. See, for example, JP-H06-65300B, JP-H03-155780A, JP-H07-289253A ([00319, [0037]), JP-H08-154672A (Claim 2 and [0027]), JP-H11-243950A ([0037]) and JP-H05-192193A.
Enzymes described in these publications are explained in brief below. Enzymes from Corynebacterium include those specific for an amino acid glycated at α-amino group but not active on fructosyl lysine (hereinafter, it may be referred to as “FL”), which are poorly heat stable (90% or more activity is decreased by treatment at 45° C. for 10 minutes) and hence lack in sufficient practical usefulness (JP-H06-65300B). Enzymes from Aspergillus include those less active on FL compared to fructosyl valine (hereinafter, it may be referred to as “FV”); however, it is unknown whether or not the enzyme is active on glycated protein or hydrolysates thereof (JP-H03-155780A). Enzymes from Gibberella include those showing high specificity to fructosyl N α-Z-lysine (hereinafter, it may be referred to as “FZL”), of which α-amino group is protected, and being active on fructosylpolylysine but not active on fructosyl valine (JP-H07-289253A, [0031] and [0037]). Enzymes from Fusarium include those having the same or higher activity for fructosyl lysine compared to fructosyl valine (JP-H08-154672A, Claim 2 and [0027]). Other enzymes from Fusarium or Gibberella include those inactive on fructosyl valine but specific for fructosyl lysine (JP-H 11-243950A, [0037]).
However, these existing enzymes are not satisfactory in terms of, for example, activity in the determination of glycohemoglobin, and therefore there has been a demand for an enzyme with high activity and excellent specificity. For instance, although these existing enzymes are active on glycated amino acids or poly-lysines produced by fragmentation with protease treatment or the like, they are almost inactive on glycated peptides in which the α-position is glycated. Accordingly, in the case of glycohemoglobin, wherein α-amino group of N-terminal amino acid is glycated, it is necessarily to release the N-terminal fructosyl valine certainly beforehand.
To measure glycated proteins accurately using an existing fructosylamino acid oxidase, it is generally inevitable to surely release the glycated amino acid as a substrate of the enzyme. However, there have not been provided any methods by which the glycated amino acid of interest can be surely released or proteases which are highly specific enough to make it sure the same. One of strategies to solve this issue is to use a fructosylamine oxidase reactive on peptide itself which is glycated at N-terminus. It is particularly important to use a fructosylamine oxidase that is also active on glycated peptides as fragmentation products so that one can measure accurately the hemoglobin A1c (HbA1c) which is significant in control of diabetes.